The invention relates to an apparatus for measuring the thickness and/or irregularities of a running sliver in a spinning preparation machine, particularly a draw frame. The apparatus is of the type which has a biased sensor element which mechanically scans (contacts) the sliver and a tongue-and-groove roll pair which defines a closed nip of generally rectangular cross section through which the sliver passes. The groove roll of the roll pair has a radially fixed rotary axis.
Published PCT Application WO-A-91 16595 discloses an apparatus for guiding the slivers at the inlet end of the drawing unit of a draw frame. The apparatus includes a conically converging sheet metal support body having laterally upwardly bent wall faces and, downstream thereof (as viewed in the direction of sliver advance), a sliver guide having a rectangular inlet cross section, parallel-extending top and bottom walls and converging, upstanding lateral walls. The side-by-side arranged slivers glide on the supporting surface formed of the supporting body and the bottom wall of the sliver guide. Between the slivers and the side walls an intermediate space is provided at the sliver intake zone. The sliver guide is situated immediately in front of a pull-off roll pair whose parallel axes are vertically oriented. The roll pair also serves for measuring the sliver thickness within a predetermined tolerance range and, for such a purpose, the distance between the two cooperating rolls of the roll pair is variable. The radially movable, spring-loaded roll forms a biased, movable sensor element and is horizontally displaceable relative to the stationary roll. The stationary roll is a "groove roll" and is composed of a middle disk and two flanking disks. The middle disk has a smaller diameter than the two flanking disks whereby the circumferential peripheral face of the roll forms a circumferential groove. The radially movable roll is a "tongue roll" and is formed of a single disk which projects, with a peripheral portion, into the groove of the groove roll. The circumferential surface of the middle disk of the groove roll forms a rotary, radially stationary counterface for the circumferential surface of the radially movable tongue roll. By means of the tongue-and-groove construction an essentially rectangular constriction (nip) is formed between which a sliver bundle formed of a plurality of slivers passes in a compressed state for measuring purposes. In operation, the individual slivers run into the sliver guide at the drawing unit inlet with a speed of, for example, 150 m/min. The converging walls of the sliver guide gather the slivers without any clamping into a single plane so that they assume a side-by-side relationship. The slivers exiting the sliver guide are first densified by being pulled into the nip of the two downstream arranged rolls, that is, they are compressed to their solid material cross section and thus, in particular, enclosed air is expelled therefrom so that a measurement may take place. The circumferential speed of the rolls and the running speed of the slivers are identical so that no slippage takes place between the rolls, on the one hand, and the slivers, on the other hand. The clamping effect of the rolls required for exerting a pulling force is simultaneously used for the densification needed for the measuring step. After the slivers exit the roll nip they diverge laterally and enter the downstream-arranged drawing unit.
It is a disadvantage of the above-outlined apparatus that it involves substantial structural and operational outlay. It is a particular drawback that the drive of the two rolls is structurally complex and also, that a rotary drive has to be used for the radially displaceable roll. It is a further disadvantage that both rolls have to be driven. The drive for the radially movable roll includes a spur gear pair; one of the gears is mounted on the shaft of the roll while the other gear is arranged coaxially with the pivot axis of the pivotal arm carrying the radially displaceable roll. This arrangement ensures that the meshing relationship of the gears of the gear pair remains unchanged independently of a pivotal motion of the pivot arm. To obtain the required, opposite rotation of the rolls, a further, intermediate gear has to be provided which has the additional disadvantage that, apart from its complex structure, clearances between the individual gear teeth lead to accumulated inaccuracies.